The invention is related to AM stereo ratio receivers. My copending patent application Ser. No. 197,294 was filed Oct. 15, 1980 (now U.S. Pat. No. 4,362,999), is titled "AM STEREO PHASE MODULATION DECODER," and is assigned to the assignee of the present invention. That application discusses AM stereo receivers and details a phase demodulator suitable for recovering the L-R channel information broadcast in AM stereo. The teaching in that application is incorporated herein by reference.
In AM stereo the sum information channel is normally amplitude modulated while the difference information channel is transmitted by phase modulation of the same carrier. In the receiver a limiter-phase detector combination is used to recover the difference information. Unfortunately, in AM broadcasting there is a tendency to operate the transmitters at a fairly high modulation level so that there are occasional overmodulation peaks. During these peaks the phase modulated carrier vanishes and the phase decoder can respond to produce noise spikes. This is particularly true of synchronous phase detectors which are provided with an IF derived carrier from a phase locked loop oscillator. The detector of my copending application Ser. No. 197,294 displays greatly reduced sensitivity to such carrier loss but is not immune thereto.
In AM radio broadcasting it has been discovered that multipath propagation can act to increase the observed degree of carrier modulation. This phenomena can act to aggravate the effects of the normally high percentage of AM broadcast modulation. The resulting increase in apparent noise in AM stereo broadcasting can constitute a problem.